•A special double powder injection lances system was developed to improve the refining efficiency during RH process.•The effect of powder injection gas flow rate on the circulation rate and mean residence time of flux was studied.•The effect of powder injection lance positions on the mean residence time of flux was analyzed.•The position that might improve the refining effect was pointed out.In this study, the gas–liquid two phase flow behavior in the vacuum vessel during the Rheinstahl-Heraeus (RH) refining process was investigated to improve the refining effect. Special attention is paid to investigate double powder injection lances system which is used to improve the refining efficiency during RH refining process. The melt flow behavior, circulation flow rate and mean residence time of the tracers with different powder injection gas flow rates and different configurations of the double powder injection lances are simulated using three-dimension numerical model based on Eulerian-Eulerian approach. The results show that, with the increasing gas flow rate of the power injection, the circulating flow rate and the mean residence time of tracer increases. It is also noted that the mean residence time of tracer varies with the different positions of the powder injection lances. It is found that there is an optimum location of the positions of the powder injection lances corresponding to the increasing of the residence time of powder and can improve the refining effect.Download high-res image (289KB)Download full-size image

•A dendritic growth model was developed considering non-equilibrium distribution coefficient.•The physical properties with temperature dependence were considered in the extended model.•The extended model can be used to non-dilute alloys and the extensions are necessary in small particles.•Microstructure of ASP30 steel was investigated using the present model and verified by experiment.We have extended the dendritic growth model first proposed by Boettinger, Coriell and Trivedi (here termed EBCT) for microstructure simulations of rapidly solidified non-dilute alloys. The temperature-dependent distribution coefficient, obtained from calculations of phase equilibria, and the continuous growth model (CGM) were adopted in the present EBCT model to describe the solute trapping behaviors. The temperature dependence of the physical properties, which were not used in previous dendritic growth models, were also considered in the present EBCT model. These extensions allow the present EBCT model to be used for microstructure simulations of non-dilute alloys. The comparison of the present EBCT model with the BCT model proves that the considerations of the distribution coefficient and physical properties are necessary for microstructure simulations, especially for small particles with high undercoolings. Finally, the EBCT model was incorporated into the cellular automaton-finite element (CAFE) model to simulate microstructures of gas-atomized ASP30 high speed steel particles that were then compared with experimental results. Both the simulated and experimental results reveal that a columnar dendritic microstructure preferentially forms in small particles and an equiaxed microstructure forms otherwise. The applications of the present EBCT model provide a convenient way to predict the microstructure of non-dilute alloys.

In this study, the microstructure evolution of rapidly solidified ASP30 high-speed steel particles was predicted using a simulation method based on the cellular automaton-finite element (CAFE) model. The dendritic growth kinetics, in view of the characteristics of ASP30 steel, were calculated and combined with macro heat transfer calculations by user-defined functions (UDFs) to simulate the microstructure of gas-atomized particles. The relationship among particle diameter, undercooling, and the convection heat transfer coefficient was also investigated to provide cooling conditions for simulations. The simulated results indicated that a columnar grain microstructure was observed in small particles, whereas an equiaxed microstructure was observed in large particles. In addition, the morphologies and microstructures of gas-atomized ASP30 steel particles were also investigated experimentally using scanning electron microscopy (SEM). The experimental results showed that four major types of microstructures were formed: dendritic, equiaxed, mixed, and multi-droplet microstructures. The simulated results and the available experimental data are in good agreement.

Based on Chou model and unreacted-core model, a new mixed rate controlling kinetic model has been derived in this paper to investigate the adsorption reaction time t for Mg-based hydrogen storage materials as a function of temperature T, particle radius R0 and reaction fraction ζ. This new model could be predigested into individual single step and mixed two-connection step equations. The characters of this new model have also been discussed. Moreover, the new model is successfully applied for a real case and results indicate that this new model works very well and could reasonably deal with complex kinetics mechanism.Highlights► Based on Chou model a new mixed rate controlling kinetic model has been presented. ► This new model could be predigested into single and two-connection mixed steps. ► All application results indicate our new model could treat complex kinetics mechanism.

The influence of casting parameters on stray grain formation of a unidirectionally solidified superalloy IN738LC casting with three platforms was investigated by using a 3D cellular automaton-finite element (CAFE) model in CALCOSOFT package. The model was first validated by comparison of the reported grain structure of Al-7%Si (mass fraction) alloy. Then, the influence of pouring temperature, heat flux of the lateral surface, convection heat coefficient of the cooled chill and mean undercooling of the bulk nucleation on the stray grain formation was studied during the unidirectional solidification. The predictions show that the stray grain formation is obviously sensitive to the pouring temperature, heat flux and mean undercooling of the bulk nucleation. However, increasing the heat convection coefficient has little influence on the stray grain formation.

The twin-roll strip casting is regarded as the most prospective technology of near-net-shape casting. The control of the grain structure is of primary importance in twin-roll strip casting because the solidification microstructure has great influence on the quality and mechanical properties of strips. In this paper, a three-dimensional cellular automation (CA) finite-element (FE) model within CALCSOFT3D package is used to simulate the microstructure of steel strip twin-roll casting. The Gaussian distribution of nucleation sites is adopted both at the mold surface and in the melt. The KGT model is used to describe the growth kinetics of dendrite tip. Then the influence of the casting conditions such as pouring temperature and heat transfer coefficient between the rolls and the solidified strip on the strip solidification microstructure is detailed investigated. The predictions show that the temperature profile and solidification microstructure are most sensitive to the pouring temperature. And with the increase of the pouring temperature, grain density obviously increases. However, the influence of three bottom cooling conditions on microstructure formation has little difference in this study.

Periodic density functional theory (DFT) and nudged elastic Band (NEB) method have been used to investigate the hydrogen desorption on MgH2(0 0 1) and MgH2(1 1 0) surface as a first step towards understanding the dehydrogenation cycle. A study of reaction barriers for three and five pathways of MgH2(0 0 1) and MgH2(1 1 0) surface have been performed. The most favorable desorption channel needs activation energies of approximately 1.60 eV with H atoms binding on the same Mg atoms on the MgH2(1 1 0) surface. Meanwhile, the MgH2 sample was prepared by ball milling treatment and Chou method was used to investigate the hydrogen desorption kinetic mechanism of MgH2. The obtained activation energies are 1.58 ± 0.6 eV for H2 recombination on the pure surface. It is suggested that the rate-controlling step is recombination of H2 on the MgH2 surface.

First-principle pseudopotential plane wave calculations and the Nudged Elastic Band method based on density functional theory (DFT) have been used in this article to study the dissociation of molecular hydrogen on a Mg(0001) surface and the subsequent diffusion of atomic hydrogen into the magnesium substrate. First, the dissociation pathway of H2 and the relative barrier were investigated. It was shown that physical adsorption rather than chemisorption of molecular hydrogen was observed in the calculation of the dissociation process of molecular hydrogen. Also, the diffusion process of atomic hydrogen on Mg(0001) was presented. The surface effect, which affected the diffusion of hydrogen obviously, was observed. Finally, comparing the values of the activation energies for the steps of dissociation, diffusion, and desorption, our calculation further showed that the dissociation of H2 and the desorption of hydride were the rate-limiting steps.